Tone control as used in most vintage radios is a relatively simple affair, often consisting of little more than a 'top cut' circuit. Although minimal in cost for the manufacturer to offer as an extra, tone controls were a good selling point. The function of these is easy to follow: 

The circuit above (left) shows a capacitor labelled C1 connected between the output valve grid input and R1, a variable resistor which becomes the manually adjustable tone control. The reactance of the capacitor - resistor combination forms a variable time-constant, progressively bypassing the higher frequency audio to ground. 

The circuit shown above (centre) is a variation of the first top cut system with the advantage of progressive top cut, increasing as the volume is reduced, with the effect of creating a bass boost at lower volume levels in compensation for the human hearing curve which is less sensitive to bass frequencies at low volume settings. This is the equivalent of what today might be termed a 'loudness' control. The switch S1 in parallel with C3 form an additional tone control, this time by switch selection. C4 is the AF coupler and has C3 in series, shorted at will by S1. With S1 closed, C3 is bypassed and tone is normal. When S1 is opened, C3and C4 together form a low value of capacitance which provides a reduction in bass response. Taken together, the two systems provide top cut, bass cut or both. The volume control will be seen to be a 'special' tapped type. Restorers know that these are almost impossible to obtain if a replacement is needed but a little time spent understanding the function of the circuit can pay dividends in that an alternative but just as effective method may be substituted that does not call for a special potentiometer.

The circuit above (right) shows another very simple form of top cut control. C6 and R5 form a feedback circuit. With R5 at maximum, no top cut occurs. Reducing the value of R5 by adjusting the slider increases the frequency-selective feedback, progressively cutting top. The value of C6 is important regarding the maximum amount of cut desired and is usually in the order of 0.002-0.005 microFarad. R5 is typically 1 to 2 megohms.

NEGATIVE FEEDBACK

Negative feedback is a method used to improve the frequency response of a given circuit and also to reduce harmonic distortion. Stability is also improved. The principle behind the process is to take a proportion of the signal from the output and feed it back in opposite phase to the input, thereby causing a reduction in the input as opposite phases cancel each other out. Obviously, feed one hundred percent back and the result would be zero output - a pointless exercise! In practice only a small percentage is fed back. 

In the diagram (right), the feedback is taken from the potential divider R4,5 which is across the secondary of the output transformer. C3 is frequency conscious, allowing more of the higher frequencies to be fed back to the tapping on the special volume control, forming a tone compensation circuit (the lower the volume, the greater the top cut effect therefore - apparently - the greater the bass boost).

An alternative circuit is shown (left) where R1 and C1 form a series connection to chassis from one side of the secondary of the output transformer. The voltage developed across C1 is fed back to the junction R2,3. R2 is the standard cathode bias resistor and C2 is its bypass capacitor.

R3 will have a low value in the order of a few tens of ohms. The result of this circuit is to feed back the higher audio frequencies more than the lower ones, resulting in a fixed bass 'boost'. 

Component values are quite critical for optimum performance. It should be apparent that the correct 'side' of the output transformer secondary winding must be chosen or the result would be positive feedback and severe instability. 

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